Problem: $ { {3} \times \left[\begin{array}{r}{3} \\ {-1} \\ {0}\end{array}\right] = {?} } $
Explanation: To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{r}{3}\times{3} \\ {3}\times{-1} \\ {3}\times{0}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{r}{9} \\ {-3} \\ {0}\end{array}\right]}$